<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    div {
      width: 200px;
      height: 200px;
      background-color: cadetblue;
      margin-bottom: 10px;
    }

    button {
      margin-left: 10px;
    }

    .change-border {
      border-radius: 50%;
    }
  </style>
</head>

<body>
  <div class="change"></div>
  <button class="btn">变回圆形</button>
  <button class="btn">变回方形<button>
</body>

</html>
<script>
  var box = document.getElementsByClassName('change')
  var btns = document.getElementsByClassName('btn')
  btns[0].onclick = function () {
    box[0].classList.add('change-border')
  };
  btns[1].onclick = function () {
    box[0].classList.remove('change-border')
  };
</script>